Sen. Mark Kirk is the first major elected official to abandon Trump after previously endorsing him.

Donald Trump’s attacks on a U.S. judge have been a bridge too far for one big-name Republican.

Sen. Mark Kirk (R-Ill.) unendorsed Trump on Tuesday, saying he “cannot and will not” vote for him, making him the first major elected official to abandon Trump after previously endorsing him.

"I have spent my life building bridges and tearing down barriers — not building walls. That's why I find Donald Trump's belief that an American-born judge of Mexican descent is incapable of fairly presiding over his case is not only dead wrong, it is un-American,” Kirk said in a statement.

The Illinois senator is facing a steep uphill reelection battle, and had previously said he’d support Trump. But he flipped to opposition over the billionaire’s latest racism tantrums.

Sen. Mark Kirk unendorsed Donald Trump (pictured) on Tuesday.

(JOSH EDELSON/AFP/Getty Images)

Kirk, a military veteran who has used a cane and wheelchair since a stroke a few year back, said he had hoped that Trump would “tone down” his rhetoric. But his latest statement, along with past attacks “on Hispanics, women and the disabled like me,” meant he can’t back the nominee because he’s shown he lacks “the temperament necessary to assume the greatest office in the world."

Kirk is the first major Republican to decide Trump has gone too far over his attacks on U.S. District Judge Gonzalo Curiel, who was born in Indiana and is of Mexican descent. Others have been severely critical, with even backers ripping him apart in recent days.